Nuclear fusion engines will be needed to power the next generation of robotic spacecraft and future human missions to Mars.
Starting up a fusion reaction requires considerable power for a short period of time. The power is used to heat the reactants to fusion temperatures until the reaction is self-sustaining. It also energizes the superconducting coils used to confine the plasma and powers auxiliary equipment.
The source of the startup power is a major concern for nuclear fusion engines. It must be reliable and available at all times. It may have to last for several years without being used.
Terrestrial fusion reactors use electric power from the grid to start their reactions. Without a source of external power a space fusion engine would need to be so reliable that it would never shutdown unless it were near an orbiting power station.
Yurash (U.S. Patent Application Publication No. 2009/0000268) discloses a fusion rocket requiring an alpha particle source to start the reaction but does not disclose a method to power the alpha particle source.
Flood (U.S. Patent Application Publication No. 2010/0264656) discloses an orbital solar station that could be used to start up a fusion reactor. Orbiting power stations would require large solar arrays and large power storage that could deliver high power in short bursts. The largest solar power system to date, on the International Space Station, would be too small to start proposed fusion engines. Even if such stations could be built, they would need to be located in orbits, such as around Mars, where spacecraft were likely to go. If the fusion engine were not operational, a separate space tug would be required to bring the power station and this would greatly limit the applicability of the fusion engine technology.
Williams, et al (Realizing “2001: A Space Odyssey”: Piloted Spherical Torus Nuclear Fusion Engine”, NASA/TM-2005-213559) discloses an onboard fission reactor for fusion reactor startup. This has the disadvantage of being heavy and dangerous due to the presence of highly radioactive material. For example, in the event of a catastrophic accident, the spacecraft may to reenter the Earth's atmosphere, spreading radioactive material over a broad area. The safety problems associated with fission reactors have prevented the widespread use of fission reactors in spacecraft.
There are numerous energy storage options that can be considered. Weinberger (U.S. Pat. No. 5,214,981) discloses a superconducting magnet bearing flywheel for energy storage. This system is an alternative to the system disclosed in this application. Its advantage over other flywheel system is that the electromagnetic bearings are superconducting and do not require power which limit the time other systems can store power. Nonetheless, it would still require that this, an active system, be kept operating for long periods of time. In addition, the large flywheels present an operational safety hazard. It is also not clear that the power to mass ratio is suitable for this application.
Fuel cells are known devices that could provide power for startup. Von Doehren (U.S. Pat. No. 3,546,019) discloses fuel cells for use as a battery. Fuel cells were used extensively in the Apollo program for electricity generation. Pettigrew (U.S. Pat. No. 5,733,421) discloses a hydrogen oxygen fuel cell. A fuel cell converts the chemical reaction into electricity directly. Fuel cells do not have the high rates of discharge required. Fuel cells could be an element of the fusion reactor start up system but by themselves are insufficient.
Batteries are a well-known electricity storage technology. Batteries, however, have very low energy densities. For example, a Cobalt Lithium Ion has an energy density of 0.875 MJ/kg. This is much too low for a fusion reactor startup. In contrast, hydrogen fuel contains 142 MJ/kg. In addition, batteries cannot discharge fast enough for this application.
Batteries or fuel cells could be supplemented by supercapacitors or supercapacitors to attain the high discharge rates. Martienssen (U.S. Patent Application Publication No. 2012/0187906) discloses the use of supercapacitors for automobiles with batteries or fuel cells. This reduces the wear and tear on batteries and enables both fuel cells and batteries to deliver high power. However, the energy density of supercapacitors is very low, 0.018 MJ/kg, and the startup system would require a large bank of supercapacitors to meet the power requirements.
Electrolysis is a well-known process for breaking water into hydrogen and oxygen. Hackmyer (U.S. Pat. No. 4,265,721, May 5, 1981) discloses a novel system using microwave energy. Such systems could be used to recover fuel for future engine startups but by themself do not comprise a solution to the problem.
If the spacecraft has multiple fusion engines that can produce power then the other engines can provide the startup power. However, this does not solve the problem of a situation in which all of the engines shut down.